The APSÛ method for process-based groundwater vulnerability assessment

Groundwater vulnerability maps can be combined with pollution hazards to assess risks of groundwater pollution. However, groundwater vulnerability maps are generally difficult to interpret because they differ according to the factors considered and the way they are combined. Here, starting from process-based concepts and criteria, a robust definition for groundwater vulnerability to pollution is discussed. A methodology is developed based on processes governing the fate of pollutants at the land surface (i.e. runoff and infiltration) and below ground (i.e. pollutant transport in the subsurface). Groundwater vulnerability is evaluated based on combination of the land surface hazard and the subsurface attenuation capacity. Land surface hazard is defined to consider direct and lateral infiltration capacity of pollutants, regardless of any subsurface attenuation capacity, which refers to any process that leads to pollutant mass reduction from the infiltration location to the water table. The concept of subsurface attenuation capacity is adapted to the case of groundwater intrinsic vulnerability assessment, considering three process-based vulnerability coefficients, which are the pollutant minimum travel time from the hazard location to the water table, the pollution duration at the water table, and the maximum concentration of pollutant discharging into the groundwater. The concepts are illustrated by applying the developed method (named APSÛ) for intrinsic groundwater vulnerability assessment in the Néblon catchment, a karstified limestone/sandstone aquifer system in Belgium. The APSÛ method results are discussed and the perspectives for generalizing the method to groundwater-specific vulnerability and risk mapping are presented.